The cell cycle regulatory protein cyclin E is overexpressed in many different cancers. The level of cyclin E in tumors is independent of the proliferation index, suggesting that its overexpression is a cause rather than an effect of tumorigenesis. In normal cells, cyclin E MRNA and protein are expressed briefly during the transition from the first gap phase (G1) to the DNA synthesis phase (S) when it binds to and activates its kinase partner, cyclin-dependent kinase 2 (cdk2). In cancer cells, high levels of cyclin E throughout the cell cycle result in sustained cdk2 activity and uncontrolled cell growth. Alterations in cyclin E expression include increased protein stability, increased stability and overexpression of mRNA and gene amplification. Our long-term goal is to define the mechanisms leading to increased cyclin E mRNA and protein stability in human cancers. The purpose of this proposal is to use Xenopus laevis embryos, a system that naturally overexpresses cyclin E, to identify novel pathways that lead to cyclin E overexpression. As in cancer cells, cyclin E overexpression in Xenopus embryos is due to increased mRNA and protein stability; however, cdk2 activity appears to be regulated despite high cyclin E levels. In addition, cyclin E protein is destabilized when cells begin the adult somatic cell cycle. We will use the Xenopus system to determine 1) how cyclin E mRNA is stabilized, 2) how high cyclin E protein levels are maintained, and 3) how cyclin E protein is destabilized at the onset of the adult cell cycle. Lastly, 4) we will ask if mechanisms regulating cyclin E are conserved between Xenopus embryos and human cancer cell lines. Once mechanisms are identified, Xenopus represents an ideal in vivo and in vitro system to identify therapeutic compounds that target elevated cyclin E and thus cdk2 activity.